Cathodoluminescent screen for presenting a differential color display and method of forming the same

ABSTRACT

A CATHODOLUMINESCENT SCREEN FORMED OF A HOMOGENEOUS MIXTURE OF TWO ELECTRON RESPONSIVE PHOSPHORS HAVING DIFFERENT SPECTRAL EMISSIONS. THE SECONDPHOSPHOR, PRESENT IN A GREATER AMOUNT, HAS EACH PARTICLE SURFACE MODIFIED TO PROVIDE A PERIPHERAL ENERGY ABSORBING BARRIER FOR ELECTRON BEAMS UNDER A PREDETERMINED THRESHOLD ENERGY. AN ELECTRON BEAM OF A FIRST ENERGY EXCITES THE FIRST PHOSPHOR, BUT A BEAM OF HIGHER ENERGY IS REQUIRED TO PENETRATE AND EXCITE THE TREATED PHOSPHOR. WHILE THE FIRST PHOSPHOR IS ALSO SIMULTANEOUSLY EXCITED, THE SECOND PHOSPHOR SPECTRAL EMISSION PREDOMINATES THE ADDITIVE MIXTURE OF COLOR EMISSION TO PROVIDE A DIFFERENTIAL COLOR DISPLAY AS THE ENERGY OF THE ELECTRON BEAM IS VARIED.

March 30, 1971 v, GALLARO 3,573,084

CATHODOLUMINESCENT SCREEN FOR PRESENTING A DIFFERENTIAL COLOR DISPLAY AND METHOD OF FORMING THE SAME Filed Feb. 25, 1968 3 Sheets-$heet 1 f f G mvsw'rox. Mn/ow Gamma 47'70RNEY March 30, 1971 v, GALLARO 3,573,084

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United States Patent US. Cl. 117-335 8 Claims ABSTRACT OF THE DISCLOSURE A cathodoluminescent screen formed of a homogeneous mixture of two electron responsive phosphors having different spectral emissions. The second phosphor, present in a greater amount, has each particle surface modified to provide a peripheral energy absorbing barrier for electron beams under a predetermined threshold energy. An electron beam of a first energy excites the first phosphor, but a beam of higher energy is required to penetrate and excite the treated phosphor. While the first phosphor is also simultaneously excited, the second phosphor spectral emission predominates the additive mixture of color emission to provide a differential color display as the energy of the electron beam is varied.

BACKGROUND OF THE INVENTION In certain types of cathode ray tube applications it is desired to present visual color displays of differing information in a distinctive and contrasting manner. One type of cathode ray tube that is applicable for the presentation of at least a two color informational display usually em ploys a layered cascade cathodoluminescent screen wherein the phosphors are excited to a state of luminescence by the penetration of electron beams of differing energies. A screen of this nature usually comprises at least two separated layers of electron responsive phosphors of different spectral emissions. Disposed intermediate the phosphor layers is a continuous and uniform layer of nonluminescent material formed to effect a discrete energy absorbing barrier for electron beams of energies below a predetermined level.

In the fabrication of a cascade screen, it is conventional in the art to form a first or inner layer of one of the phosphors on the interior surface of the viewing portion of the tube envelope, such as by the known settling technique. Upon this first screen layer, the barrier layer is applied in a manner to cover the whole of the surface thereof. Suitable electron energy absorbing barrier materials are, for example, silicon dioxide, zirconium silicate, cadmium oxide, or other appropriate non-luminescent substances that lend themselves to conventional deposition, such as by an evaporation technique. In forming small screens, for example, those having diametrical dimensions of under ten inches, an energy absorbing barrier layer of substantially uniform thickness can be adequately achieved; but in the fabrication of screens of a larger size, difficulty has been experienced in disposing an overall layer of uniform thickness by evaporation as the screen area nearest the source of evaporable material receives a greater thickness of deposition. Therefore, when a larger size screen having a layer of a second phosphor subsequently applied over the aforementioned barrier layer, is formed according to the above manner, the excited color response from the finished screen is unpredictable due to the varying thickness and nonuniformity of the barrier material therein.

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OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention to reduce the aforementioned difficulties and to provide an improved cathodoluminescent screen capable of presenting substantially uniform and distinctive color responses when excited by electron beams of predetermined energies.

Another object is to provide an improved screen exhibiting substantially uniform color response regardless of the size of the screen.

A further object is to provide a method of forming an energy responsive cathodoluminescent screen that enables repetitive fabrication of a uniform screen product.

The foregoing objects are achieved in one aspect of the invention by the provision of an improved cathodolurninescent screen wherein a homogeneous mixture of at least first and second electron responsive phosphors, having different spectral emissions, is disposed as a single uniform layer. The first phosphor of the homogeneous mixture, when excited by an electron beam of a first energy, emits luminous energy of a first spectral emission. The second phosphor in the mixture is treated in a predetermined manner prior to screening whereby the peripheral portion of each phosphor particle is modified to be non-responsive to electron excitation to a substantially predetermined depth. Thus, an integral efficiency reducing encasement is provided on each particle to effect an energy absorbing barrier for electron beams under a predetermined threshold energy. Since this selectively treated second phosphor is present in the screen mixture in a major predetermined amount, the spectral emission therefrom predominates the additive combination of color emissions thereby providing a differential color display which changes as the energy of the exciting electron beam is varied.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 and 2 are enlarged partial cross-sectional views of the cathodoluminescent screen illustrating construction of the screen and excitation thereof by low and high energy electron beams respectively;

FIG. 3 is a standard C.I.E. chromaticity diagram illustrating the relationship of the color-emitting phosphors in one embodiment of the invention; and

FIG. 4 is a spectral emission curve of the excited screen.

DESCRIPTION OF THE PREFERRED EMBODIMENT For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following specification and appended claims in connection with the aforedescribed drawings.

With reference to the drawings, there is shown in FIG. 1 a cross-sectional view of the cathodoluminescent screen 11 of the invention which is suitably disposed on an appropriate substantially transparent glass substrate 13, as for example, the face panel portion of a cathode ray tube, additional details of which are not shown. The screen 11 is comprised of a homogeneous mixture of at least two electron responsive phosphor compositions having similar ranges of particle sizes but different spectral emissions, i.e., a first phosphor 15 and a second phosphor 17. The second phosphor is treated, prior to screen forming, in a manner that the peripheral structural portion 19 of each phosphor particle is altered to a predetermined depth to be non-responsive to electron excitation. This peripheral modification of the second phosphor forms an integral energy absorbing electron barrier on each particle that prevents electron beams of under a certain energy from penetrating the particle and exciting the interior or unmodified portion 21 thereof. The two phosphors in the homogeneous screen mixture are predeterminately proportioned whereby the treated second phosphor 17 is present in the greater amount.

A low energy electron beam 23, from an electron gun source not shown, is beamed to impinge the screen 11, whereupon the first electron responsive phosphor particles are excited to luminescence. While the excited phosphor particles emit light in all directions, for purposes of clarity of illustration, only the light rays 25 projecting outwardly from the screen are shown in the figure. The impinging low energy electron beam 23 is substantially absorbed by the treated non-responsive peripheral portion of each second phosphor particle. Thus, under low energy excitation only the spectral emission of the first phosphor is evidenced.

In FIG. 2, a high energy electron beam 27 emanating from an electron gun source not shown, is beamed to impinge the phosphors of the screen 11. Both phosphors are excited to luminescence, the higher beam energy being suflicient to penetrate the peripheral energy absorbing barrier 19 of the treated second phosphor particle 17 and excite the interior portion 21 thereof to produce light rays 29 of a second spectral emission. Since the screen mixture has a larger amount of the second phosphor therein, the spectral emission therefrom influences the combined spectral emission color display 31 resultant from high energy excitation. The ratio of the first phosphor to the treated second phosphor in the mixture is at least 1:2, the specific ratio being determined by the inherent luminance efliciencies of the phosphors.

In greater detail and by Way of an illustrative example, the homogeneous mixture comprising cathodoluminescent screen 11 has therein a red-emitting first phosphor 15 and a green-emitting second phosphor 17. The respective hues of these spectral emissions have the capabilities for presenting a distinctive differential color display falling in that portion of the spectrum wherein the human eye is particularly sensitive. In the described embodiment, the red-emitting first phosphor particles 15 in the homo geneous screen mixture may be, for example, europium activated yttrium vanadate phosphor material having a white body color, whereof the crystals have a white appearance and a high degree of translucency.

The treated second phosphor component 17 of the mixture may be, for example, green-emitting silver activated zinc-cadmium sulfide having substantially a white body color. In this instance, the second phosphor 17 has a luminance efliciency of a level higher than that of said first phosphor 15. As previously mentioned, the peripheral portion of each second phosphor particle is discretely modified, prior to screening, to effect an energy absorbing region of reduced efficiency that is an integral portion of the phosphor crystal. This is accomplished by carefully diffusing a selected metallic material into the surface structure of each particle. Metals suitable for such incorporation may include one or more of the group comprising cobalt, iron, or nickel; the application of which may be in the form of nitrates, chlorides, or sulfates. For example, a suspension of 50 grams of green-emitting silver activated zinc-cadmium sulfide phosphor material is made in 225 milliliters of Water with 12.5 milliliters of 0.006 molar cobalt nitrate added thereto. The suspension is agitated while 25 milliliters of .015 molar ammonium sulfide is slowly introduced to form cobalt sulfide which adheres to the surface of the particles. The unattached cobalt material is removed by washing the particles with a ketone such as acetone; after which they are dried and substantially size-graded in preparation for subsequent firing whereby the adhered cobalt material is heat diffused into the peripheral portion of each phosphor particle. The depth of diffusion is controlled by a time-temperature relationship in the presence of an inert atmosphere, such as nitrogen. For example, firing the treated phosphor at substantially 800 degrees C. for 11 minutes provides an energy absorbing barrier of sufficient depth to effect a threshold energy factor of approximately 6 kv. Increasing the firing temperature to substantially 880 degrees C. deepens the cobalt diffusion into the particle thereby effecting an energy threshold of approximately 12 kv.

In the homogenous screen mixture of the example, the first phosphor and the treated second phosphor particles are of substantially similar size ranges and densities. Thus, the individual particles of each phosphor are substantially similar both by weight and volumetrical considerations. To promote a single-layer homogeneous screen structure of desired luminance and uniformity, wide ranges and small sizes of particles should be avoided to prevent dense packing and cascade layering of the phosphors. For example, when 6 microns is considered as an average size of a used range, all of the particles should be of a size substantially in excess of 2 microns. In order to attain the desired color display from the finished screen the homogeneous mixture of phosphors is proportioned with the treated second-green emitting phosphor being present in a greater amount. It has been found that an approximate ratio of 1:4 is quite satisfactory for the phosphors concerned from the standpoint of both weight and volume. For example, in screening a tube having a ten inch diametrically dimensioned screen, approximately 0.75 gram of the red-emitting yttrium vanadate material and approximately 3.00 grams of the green-emitting zinc-cadimum sulfide phosphor are used. These two phosphor materials are homogeneously disposed as a one-layer screen by one of several conventionally known techniques such as, for example, the barium actate-potassium silicate settling system. By utilizing the basics of this method, a potassium silicate-phosphor suspension is formed by homogeneously combining the 3.75 grams of the two phosphors with substantially 360 milliliters of .540 normal potassium silicate. It has been found that this suspension can be formed in either of two ways; first, by separately introducing the desired amounts of the respective phosphors into the potassium silicate which is agitated to effect homogenuity, or secondly, by thoroughly dry blending the proportioned phosphors prior to formation of the suspension. Upon formation, the suspension is dispensed into the bulb containing a liquid cushion comprised of milliliters of 1 percent barium acetate and 3000 milliliters of water. After a settling time of approximately 25 minutes, the liquid is carefully decanted from the bulb, and the screen formed therein is air dried. Subsequently, the screen is baked at substantially 400 degrees C. for approximately 40 minutes to remove the volatile materials therefrom. If desired, aluminizing may be conventionally applied thereover. The screen so formed has a phosphor screen weight of substantially 8.7 mg./cm. wherein the first red-emitting phosphor contributes substantially 1.7 mg/cm. and the second green-emitting phosphor substantially 7.0 mg./ cm. The volumetric ratio of the first and second phosphors in the screen is also substantially l1 :4.

By way of example, in describing the color display of the screen 11, the green-emitting second phosphor 17 will be considered to have an electron energy absorbing barrier effecting an energy threshold of approximately 6 kv. The low energy electron beam 23, being under 6 kv. excites the yttrium vanadate red-emitting first phosphor 15. The red light rays 25 emanating therefrom present a red display to the viewer facing the substrate or panel 13. As the voltage of the electron beam is increased to the energy threshold of the treated green-emitting second phosphor 17, there is slight pentration of the energy-absorbing barrier 19. This results in a limited degree of excitation of the second phosphor wherefrom a faint green luminescence is evidenced which admixes with the already present red emission. As the energy threshold is passed, the green phosphor becomes more fully excited and a greater amount of green color light is emitted. When the high energy beam 27 is in excess of 6 kv. all of the treated second phosphor 17 becomes luminous; and since there is a greater amount of green-emitting phosphor, the spectral emission therefrom shifts the color admixture 31 of the display to the green-yellow region of the spectrum.

To further describe the color display of the homogeneous screen structure, reference is made to FIG. 3 wherein there is shown a standard C.I.E. (Conference International dEclairage) chromaticity diagram having color definitive x and y coordinates whereby color hue and degree of saturation may be designated. The boundary of the horseshoe-shaped figure defines a monochromatic locus of which the blue and red portions of the spectrum are joined across the bottom by a substantially straight line forming a locus of pure purple. Encompassed within the loci of the diagram is the range of colors to which the normal human eye is sensitive. A sequential series of numbers, adjacent to the loci, indicate pure spectral wavelengths in millimicrons (mu). By these designations, colors can be defined in terms of the x and y coordinates of the diagram. A conventional reference point or one designation of relative white within the diagram is noted as Illuminant C. and is indicated on the dagram as point C at the intersection of x coordinate 0.310 and y coordinate 0.316.

In the described screen embodiment as portrayed in FIG. 3, the red-emitting first phosphor 15 is plotted at R by coordinates x 0.660 and y 0.335 and the green-emitting treated second phosphor 17 at G by coordinates x 0.265 and y 0.590. The designation R of the red-emitting first phosphor lies within the diagram, but is near the locus thereof which indicates it to be a nearly saturated color, i.e., it contains little white light. The G designation of the green'emitting second phosphor is farther removed from the locus indicating that it contains more white light and therefore is a less saturated color. Two straight lines CR and CG are extended from C through points G and R respectively to intersect the monochromatic locus at points R and G. The R point of intersection indicates the dominant wavelength of the first phosphor 15 to be approximately 615 me, while the point G denotes the dominant wavelength of the second phosphor 17 to be approximately 540 tri A line drawn between points R and G signifies the path of color shift during screen excitation. When the screen is impinged by a low energy electron beam, the first phosphor is excited whereby luminescence of the color R is evidenced. An increase in beam energy to exceed the 6 kv. threshold voltage of the second phosphor continues excitation of the first phosphor and also initiates excitation of the treated second phosphor in the screen mixture. The green luminescence thus produced mixes with the already present red emission and produces a differential color display or shift along line GR toward point G. Since red emission is always present in the display, the color shift resultant of the two excited phosphors never reaches point G but, upon full excitation, does reach a compromise point A on the line GR. It has been found that point A in the described embodiment has an x coordinate of 0.378 and a y coordinate of 0.517. A line CA drawn from C through A to intersect the locus at A indicates the dominant wavelength of the visual display when both phosphors are fully excited to be a greenyellow of substantially 5 64 m A spectral energy distribution (S.E.D.) is shown in FIG. 4 wherein the relative radiant energies of the excited phosphors are related to their respective wavelengths. A spectral energy curve 33 indicates the color emission from the first phosphor at 6 kv. low energy electron beam excitation. Another spectral energy distribution curve denotes the respective emissions of the two phosphors at 15 kv. high energy beam excitation.

At 15 kv. the higher energy beam manifests the increased radiant energy emission or brightness of the red and excites the green phosphor component to full brightness.

A standard visibility curve 39 is superimposed in a relative manner over the S.E.D. curves 33 and 35. This indicates substantially what the human eye actually sees. The eye does not differentiate between the red 39 and the green 41 portion of the curve 35, but makes a composite spectral interpretation of all color emissions observed. The fully excited spectral information portrayed in FIG. 4 is interpolated to produce the combined evaluation by the human eye as the x and y coordinates of A in FIG. 3.

The differential color display produced by the homogeneous screen mixture of the invention is resultant of the energy of the impinging electron beam or beams. A low energy beam excites only the first phosphor, as at R, while an optimum high energy beam fully excites both phosphors, as at A. Beam energies therebetween excite both color emissions with the admixture thereof being evidenced along the line AR in FIG. 3. When more than one electron gun is utilized whereby the screen is impinged by different voltage beams, a display of two or more colors of AR line characteristics can be viewed on the screen simultaneously. The white body color of the phosphors markedly contributes to the luminous transmission of the screen and enhances the color display thereof.

The homogeneous screen of the invention is not limited to the particular phosphors or color emissions described.

The cathodoluminescent screen of the invention exhibits substantially uniform color response regardless of the size of the screen. The homogeneous mixture of phosphors provides a uniformity of manufacture not conventionally evidenced in cascade type screens. The combined phosphors and discrete proportions of the invention provide a single layer homogeneous screen structure that is capable of producing a distinctive and differential color display.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

I claim:

1. A cathode ray tube for presenting a differential color display having therein at least one source of electrons capable of emitting at least a first and a second electron beam of defined differing energies and a cathodoluminescent screen comprising:

a screen layer of a homogeneous mixture of at least first and second electron responsive phosphor compositions of different spectral emissions; said first and second phospor particles being of similar weight and volumetrical considerations; said phosphors being chosen, selectively treated and proportioned by volume relative to one another wherein the proportions of said first phosphor to said second phosphor is at least 1:2; said first phosphor being of a composition to produce a first spectral emission of luminous energy when excited by an electron beam of a first energy; and said second phosphor being in the form of phosphor particles of a priorly treated composition whereof the peripheral portion of each particle structure is modified with at least one metallic diffusion selected from the group consisting of cobalt, iron, and nickel to effect an electron efliciency reducing encasement therearound, the depth of said diffusion being determined by firing at a time-temperature relationship; said treated second phosphor when penetrated and excited by an electron beam of a second energy higher than said first energy produces a second apparent intrinsic spectral emission range formed'of an additive mixture of luminous energies of the excited first and second phosphors to provide a differential color display of the spectral emissions of said phosphors when excited by electron beams of differing energies.

2. A cathodoluminescent screen according to claim 1 wherein said first and second phosphors are of similar particle sizes and densities and wherein the ratio of said first phosphor to said treated second phosphor contained in said screen is of an amount to provide a predominant second phosphor spectral emision influence on the combined color display resultant of said high energy excitation, the amount of said treated phosphor contained in said screen being determined by the inherent luminance efiiciency of said phosphor.

3. A cathodoluminescent screen according to claim 1 wherein said differential spectral emission display has dominant wavelengths relative to C.I.E. illuminant C ranging substantially between 540 :and 620 millimicrons.

4. A cathodoluminescent screen according to claim 1 wherein said treated second phosphor composition has a luminance efliciency of a level higher than that of said first phosphor composition.

5. A cathodoluminescent screen according to claim 1 wherein said homogeneous mixture is comprised of a substantially red emitting phosphor composition and a substantially green emitting phosphor composition, and wherein the phosphor particles are of substantially white body color and of substantially similar densities.

6. A method of disposing a homogeneous mixture of first and second electron responsive phosphors to form a single layer cathode ray tube screen capable of presenting a differential cathodoluminescent color display resultant of the excitation of said phosphors by electron beams of different energies and wherein the spectral emission of said first phosphor is present in all of said display presentations, said method comprising the steps of:

treating said second phosphor whereof the eificiency of the peripheral portion of each particle structure is modified by diffusing at least one specific metallic material into the surface thereof at a time-temperature relationship in an inert atmosphere to effect a specific threshold for electron energy penetration, said diffusive material being selected from the group consisting of cobalt, iron and nickel;

forming a suspension of said first and said treated second phosphor in potassium silicate as a homogeneous mixture wherein the proportions of said first phosphor to said second phosphor is at least 1:2, said phosphors being of similar densities and particle slzes;

dispensing said suspension into the bulb of said tube containing a liquid cushion of barium acetate and water;

settling said homogeneous mixture of particles to form a homogeneous single layer screen; and

drying and baking said screen to remove the volatile screening materials therefrom.

7. A method of forming a homogeneous single layer cathodoluminescent screen according to claim 6 wherein said first and second phosphors are dry blended in at least a 1:2 proportion prior to being introduced into said potassium silicate material.

8. A method of forming a homogeneous single layer cathodoluminescent screen according to claim 6 wherein said first and said second phosphors are combined in said homogeneous mixture in a substantially 1:4 proportion.

References Cited UNITED STATES PATENTS 2,996,380 8/1961 Evans 96-361 ALFRED L. LEAVITT, Primary Examiner W. F. CYRON, Assistant Examiner U.S. Cl. X.R. 

